Turbine engines commonly operate at efficiencies less than the theoretical maximum because, among other things, losses occur in the flow path as hot compressed gas travels down the length of the turbine engine. One example of a flow path loss is the leakage of hot combustion gases across the tips of the turbine blades where work is not exerted on the turbine blade. This leakage occurs across a space between the tips of the rotating turbine blades and the surrounding stationary structure, such as ring segments that form a ring seal. This spacing is often referred to as the blade tip clearance.
Blade tip clearances cannot be eliminated because, during transient conditions such as during engine startup or part load operation, the rotating parts (blades, rotor, and discs) and stationary parts (outer casing, blade rings, and ring segments) thermally expand at different rates. As a result, blade tip clearances can actually decrease during engine startup until steady state operation is achieved at which point the clearances can increase, thereby reducing the efficiency of the engine.
Although control systems have been developed to address the differences in blade tip clearance throughout the operational state of the turbine engine, inefficiencies still exist. Other structural improvement to blade tips and/or blade ring seals have not eliminated the inefficiencies. Thus, there is a need for reducing leakage past turbine blade tips in order to maximize the efficiency of a turbine engine.